Using Layer 2 Traceroute; Layer 2 Traceroute Usage Guidelines; Identifying A Layer 2 Path - Cisco WS-C2950SX-48-SI Configuration Manual

Chapter 20 Checking Status and Connectivity

Using Layer 2 Traceroute

The Layer 2 Traceroute utility allows you to identify the physical path that a packet takes from a source
to a destination. This utility determines the path by looking at the forwarding engine tables of the
switches in the path.
Information is displayed about all Catalyst 4500 series, Catalyst 5000 family, and Catalyst 6500 series
switches that are in the path from the source to the destination.

Layer 2 Traceroute Usage Guidelines

This section lists the guidelines for the Layer 2 Traceroute utility:
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Identifying a Layer 2 Path

To identify a Layer 2 path, perform one of these tasks in privileged mode:
Task
Trace a Layer 2 path using MAC
addresses.
Trace a Layer 2 path using IP addresses or
IP aliases.
This example shows the source and destination MAC addresses specified, with no VLAN specified but
with the detail option specified. For each Catalyst 4500 series, 5000 family, and 6500 series switch found
in the path, the output shows the device type, device name, device IP address, in port name, in port speed,
in port duplex mode, out port name, out port speed, and out port duplex mode.
78-15486-01
The Layer 2 Traceroute utility works for unicast traffic only.
You must enable CDP on all of the Catalyst 4500 series, Catalyst 5000 family, and Catalyst 6500
series switches in the network. (See
CDP.) If any devices in the path are transparent to CDP, l2trace will not be able to trace the Layer 2 path
through those devices.
You can use this utility from a switch that is not in the Layer 2 path between the source and the
destination; however, all of the switches in the path, including the source and destination, must be
reachable from the switch.
All switches in the path must be reachable from each other.
You can trace a Layer 2 path by specifying the source and destination IP addresses (or IP aliases) or
the MAC addresses. If the source and destination belong to multiple VLANs and you specify MAC
addresses, you can also specify a VLAN.
The source and destination switches must belong to the same VLAN.
The maximum number of hops an l2trace query will try is 10; this includes hops involved in source
tracing.
The Layer 2 Traceroute utility does not work with Token Ring VLANs, when multiple devices are
attached to one port through hubs, or when multiple neighbors are on a port.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
Chapter 21, "Configuring CDP,"
Command
l2trace {src-mac-addr} {dest-mac-addr} [vlan] [detail]
l2trace {src-ip-addr} {dest-ip-addr} [detail]
Using Layer 2 Traceroute
for information about enabling
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Release 8.1
20-11